A general air conditioner for a vehicle includes, for example, a compressor, a condenser, an evaporator, etc. constituting a refrigerant cycle. The vehicle air conditioner also includes an air conditioning body unit for air-conditioning a vehicle compartment, a blower unit including a blower, a vehicle air-conditioning air duct for guiding air from the air conditioning body unit to the vehicle compartment, and an outlet port for discharging the conditioned air from the air duct into the vehicle compartment.
An air duct for vehicle air conditioning is disclosed in Japanese Laid-open Patent Publication No. 2001-277836. As the air duct, an air guide plate is installed to divide an air flow passage into inner and outer peripheral sides at a corner of the air flow passage. In accordance with the installation of such an air guide plate, an air flow biased toward the outside of a bending section at the corner is corrected.
In an air conditioner for a vehicle, air flows through a blower unit, an air conditioning body unit, an air duct for vehicle air conditioning, and an outlet port in a sequential order. As a result, noise generated due to a sound of the blower and a flow of air in the air conditioning body unit is propagated through the vehicle air-conditioning air duct, so that the noise is emitted to the vehicle compartment. In conventional cases, for example, a measure has been taken against noise in a manner of that a sound-absorbing materials is installed in the air conditioning body unit or the air duct for vehicle air conditioning.
Meanwhile, in fields other than vehicle air conditioners, an interference type sound attenuator is used as a measure against noise, as described in “Noise Attenuation Chapter of Technology and Law for Environmental Pollution Prevention” (edited by the editing committee of Technology and Law for Environmental Pollution Prevention, supervised by Environmental Location Bureau, Ministry of International Trade and Industry, published by Corporation Aggregate Industrial Environmental Management Association, and sold by Maruzen Co., Ltd.). A schematic diagram of such an interference type sound attenuator is shown in FIG. 55. The sound attenuator shown in FIG. 55 includes a bypass passage 502 for traveling a sound while bypassing the sound from a rectilinear main passage 501. The length difference between the bypass passage 502 and the main passage 501 from a branch point 503 to a junction point 504, i.e., a passage length difference “L10−L20”, is set to correspond to an integer-number multiple of the half wavelength of the waveform of the sound, in order to attenuate the sound by using a sound wave having a phase opposite to the phase of the sound to interfere with the sound in a junction portion.
Japanese Laid-open Patent Publication No. 2004-196180 also discloses a duct, which has a function of the above-mentioned interference type sound attenuator, and is used as an air inlet passage of an engine. In Japanese Laid-open Patent Publication No. 2004-196180, however, there is no disclosure as to a specific application example of an interference type sound attenuator to an air duct for vehicle air conditioning.
Japanese Laid-open Patent Publication No. 2003-194018 discloses a flow guide blade-inserted elbow for use in a cavitation tunnel or a wind tunnel. Although this elbow does not function as an interference type sound attenuator, it causes a flow of fluid to be constant by virtue of flow guide blades installed in the duct.
Recently, air conditioning capable of maintaining the vehicle compartment in a silent state has been required in accordance with an improvement in vehicle quietness. As mentioned above, noise emitted from an air conditioning body unit to the vehicle compartment is mainly generated in a wide region at the upstream side of an air flow, rather than at an outlet port. For example, such noise is generated while the blower operates or while air passes through an evaporator, etc. Thus, when a noise attenuating function is provided to an air duct for vehicle air conditioning connected to an outlet port, as a measure against noise, the measure is effective for all noise generated in a region arranged upstream of an air flow rather than the outlet port.
Accordingly, the inventors of the present application examined the provision of an interference type noise attenuating function to an air duct for vehicle air conditioning.
First of all, an effective noise reduction is achieved when a noise target range to be attenuated ranges from a frequency of 0.8 kHz to 3.5 kHz in an air duct for vehicle air conditioning having an interference type noise attenuating function. In particular, the inventors found, from the results of examination, that noise having a frequency ranging from 1 kHz to 2.5 kHz can be very effectively attenuated.
The above-mentioned examination results are shown in FIGS. 56A and 56B. FIG. 56B shows the frequency characteristics of an actual vehicle blowing noise. FIG. 56A shows the results of an examination about how a person senses noise having frequency characteristics identical to those of the actual vehicle blowing noise shown in FIG. 56B and noise obtained after sampling noise having a ⅓ octave analysis frequency from the former noise, lowering the sampled noise to 3 dB.A, and equalizing the resultant noise to the former noise, when the two noises are compared to each other. In FIG. 56A, “O” represents that a person definitely senses quietness, “□” represents that a person indefinitely senses quietness to a certain degree, and “X” represents that a person senses no difference between the two noises. As shown in FIG. 56A, the person can sense quietness in the frequency band ranging from 0.8 kHz to 3.5 kHz after listening to and comparing the two noises in accordance with a level reduction of one noise as compared to the other noise. Also, the person can sense quietness more clearly in the frequency band ranging from 1 kHz to 2.5 kHz.
On the other hand, the conventional measure against noise in a vehicle air conditioner, using a sound-absorbing materials, etc., is effective to noise of a high frequency band higher than 2 kHz. In this case, however, there is a problem in that the effect decreases on noise reduction of a low frequency band of 2 kHz or less.
It is required to achieve a noise reduction effect for noise of a frequency band of 0.8 kHz to 2 kHz, more preferably, 1 kHz to 2 kHz, in which an interference type noise attenuating function is provided to an air duct for vehicle air conditioning.
Meanwhile, since sound waves have directionality, it is difficult for sound waves to travel in a direction normal to an original travel direction. As described in “Noise Attenuation Chapter of Technology and Law for Environmental Pollution Prevention”, it is impossible to obtain a desired noise reduction effect using interference because a sound mainly flows through the main passage 501 shown in the configuration in FIG. 55, in which the bypass passage 502 is branched from the rectilinear main passage 501 in a normal direction. In this case, even if a noise reduction effect is obtained, it is small. As a measure against such a problem, the main passage 501 may be squeezed, i.e., may have a reduced cross-sectional area, at a portion arranged downstream of the branch point thereof, such that the sounds flowing through the main passage 501 and the bypass passage 502 are equalized in energy level. In this case, however, there is a problem in that an air flow resistance increases due to the reduced passage cross-sectional area.
When a bypass passage is provided at a rectilinear section of the air duct for vehicle air conditioning, as shown in FIG. 55, there is a problem in that the air duct is excessively enlarged due to an increase in the region where bypass passages are formed.
Therefore, it is undesirable to provide a passage branching configuration at the rectilinear portion of an air duct for vehicle air conditioning, which constitutes a passage for conditioned air as shown in FIG. 55.
Furthermore, in the duct disclosed in Japanese Laid-open Patent Publication No. 2004-196180, the flow directions of air and sound are opposite to each other. This duct has an inlet port enlarged in size to suck air. Such a shape of the duct is peculiar, as compared to air ducts for vehicle air conditioning. Thus, there is a problem in that the duct having such a shape cannot be used for an air duct for vehicle air conditioning.